The invention relates to displays, and more particularly, to a compact illumination and projection system that may be used with a reflective LCD microdisplay.
Projection display technology has found increasing acceptance with many technologies. Projection displays modify light from light source to form a desired image and project the desired image, using optical elements such as lenses and mirrors, so that the image may be viewed by a viewer. The projection display may project the desired image onto a wall, for example, or onto a transparent screen so that a viewer on the opposite side of the screen may view the image. Such a transparent screen is used in some projection televisions and avionics displays. Projection displays cost much less than typical flat-panel displays, and are much easier to change as formats and other performance requirements change. Additionally, projection displays, when projected onto a screen, may be set to any desired size by adjusting the position and optical power of the optical elements contained therein. A single projection display xe2x80x9cenginexe2x80x9d can therefore be used in a wide variety of display formats.
LCD-based projection engines require polarization sensitive optics to discriminate between the light and dark pixels. There are two main types of LCD devices: transmissive devices and reflective devices. Transmissive devices normally have sheet polarizing filters attached to the front and back surfaces and discriminate the light and dark pixels. In contrast, reflective LCDs are illuminated and observed from the same side and therefore require a reflective/transparent polarizing optic to discriminate between the illumination beam and the projected image beam. These polarizing optics are commonly called polarizing beam splitters (PBS), and consist of two glass prisms bonded together to form a cube, with a thin film coating on the bonded interface.
High performance PBSs are bulky and expensive, especially in full color projection designs where three LCDs and three PBSs are used. PBSs also suffer from limited spectral and angular operating ranges, which impacts overall projector performance in terms of size, brightness, contrast, and color.
The drawbacks of using PBSs in an LCD-based optical system have been recognized, and attempts have been made to design an optical system that does not require one or more PBSs. One solution, shown in U.S. Pat. No. 6,076,931 issued to Bone et al. and incorporated by reference herein in its entirety, uses multiple lens groups to focus light on a microdisplay and project a desired image. Because this solution illuminates the reflective microdisplay at an angle (a de-centered design), the lenses in the lens groups are more complex to design and construct than would be required if the microdisplay is illuminated and reflects light substantially orthogonal to its surface (an on-axis design).
It is therefore an object of this invention to provide the most compact and inexpensive optical system while preserving or improving system performance.
It is another object of the invention to provide a display system that is compact enough to be used in environments where the size of the display system is a primary design criterion.
A feature of the invention is a projection display system including at least one optical element that simultaneously focuses and directs oppositely oriented beams of light.
An advantage of the invention is a projection display system having improved optical performance because of the elimination of the PBS.
Another advantage of the invention is a projection display system having a reduced size and component count when compared to other display systems with similar performance characteristics.
The invention provides an on-axis display system for displaying an image on a projection target. The display system includes a light source and a mirror that reflects light from the light source in a first direction. The mirror substantially maintains the polarization orientation of light reflected thereupon. A reflective microdisplay modifies light from the light source to form a desired image. A first plurality of lenses focuses and directs light from the light source to the microdisplay. A second plurality of lenses focuses and directs the desired image from the microdisplay to the projection target. At least one lens in the first plurality of lenses also directs the desired image from the microdisplay to the projection target.
The invention also provides an on-axis optical projection system that displays an image on a projection surface. The optical projection system includes a light source and a reflective LCD microdisplay that modifies light from the light source to form a desired image. The LCD microdisplay reflects light toward the projection surface. A mirror reflects light from the light source in a first optical path toward the LCD microdisplay. The mirror does not affect the polarization orientation of light reflected thereupon. A plurality of lenses are positioned within the system to focus and direct light along the first optical path and along a second optical path from the microdisplay to the projection surface. The first optical path is substantially opposite in direction to the second optical path. The plurality of lenses includes a dual-purpose lens that simultaneously transmits light along both the first optical path and the second optical path.
The invention further provides a method of projecting an image on a projection surface. According to the method, a light source is provided. Light from the light source is focused along a first optical path, toward a reflective LCD microdisplay, using an optical device that maintains the polarization orientation of light incident thereupon. The reflective LCD microdisplay is used to modify the polarization state of at least part of the light travelling along the first optical path. Light reflected off the microdisplay is directed along a second optical path. The first optical path and the second optical path define substantially opposite directions through a lens assembly. Light is focused toward the projection surface along the second optical path.